Recirculation system and method for vapor generator



Jun 1964 w. w. SCHROEDTER ETAL 3,135,249

RECIRCULATIO N SYSTEM AND METHOD FOR VAPOR GENERATOR 2 Sheets-Sheet 1 Filed July 27, 1961 I mu'uul'l-l I FIG.|

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AGENT u 1954 w. w. SCHROEDTER ETAL 3,135,249

RECIRCULATION SYSTEM AND METHOD FOR VAPOR GENERATOR Filed July 27, 1961 2 Sheets-Sheet 2 INVENTORS. WILLBURT W. SCHROEDTER EDWARD L. KOCHEY JR. 0w

AGENT 2 F 9 m A MA mm M \M A a D I E S R w D mm M L T L 5 UF E c c R w A N m F E R RR F 2 w m w H A 3 A R w A' mtnmmwma a mmimmmflm United States Patent 7 3,135,249 RECIRCULATION SYSTEM AND METHOD FOR VAPOR GENERATOR Willburt W. Schroedter, West Hartford, and Edward L.

Kochey, J12, Colebrook, (301111., assignors to Combustion Engineering, Inc., Windsor, Conn, a corporation of Delaware Filed July 27, 1961, Ser. No. 127,246 3 Claims. (Cl. 122--406) This invention is related to a modified once-through flow vapor generator, and more particularly to the fluid recirculating system as applied to a once-through flow vapor generator when operating in the subcritical or supercritical pressure range.

More specifically the invention is concerned with improving the operation of the recirculating pump system when a reserve recirculating pump is provided and/or a recirculating pump bypass, both being arranged in parallel flow with the recirculating pump being in operation.

It is a main object of this invention to improve the economy of the recirculation system both as to maintenance requirements and as to method of operation.

It is another object of the invention to reduce outage time of the vapor generator as it may be required for the maintenance of the recirculating system.

A still further object of the invention is to increase the life of the recirculating pumps and of the check valves 1which are provided in the pump lines and in the bypass Other and further objects of the invention will become apparent to those skilled in the art as the description hereof proceeds.

With the aforementioned objects in view, the invention compromises an arrangement, construction and combination of the elements of the inventive organization in such a manner as to attain the results desired as hereinafter more particularly set forth in the following detailed description of an illustrative embodiment, said embodiment being shown by the accompanying drawings in which:

FIG. 1 represents a diagrammatic elevational view of a vapor generator incorporating the improvement;

FIG. 2 is a sectional and more or less diagrammatic view of a recirculating pump of the submerged motor type, having bearings lubricated by water;

. FIG. 3 is a diagram showing the system resistance and pump system head plotted against flow capacity with one pump curve indicating a negative head, and another curve indicating a positive head when operating at vapor requirements of the vapor generator above the recirculation run-out requirement;

FIG. 4 is a flow diagram of the improved vapor generating and recirculating system;

FIG. 5 shows a set of curves illustrating the flow resistances of the working fluid as it passes through the heating surfaces and when employing a recirculating pump conventionally producing a negative pressure head when the vapor generator is operating in the load range above the load at which recirculation ceases; and

. FIG. 6 is a set of curves similar to those of FIG. 5 illustrating the flow resistance of the working fluid as it passes through the heating surfaces and when employing a recirculating pump system having a positive pressure head throughout the upper vapor generator load range, above the load at which recirculation ceases.

In the operation of once-through flow vapor generators experience has dictated that a minimum rate of circulation of the water in the heating tubes must be maintained at all times to prevent overheating of these tubes during the operation of the unit in a load range below the maximum load. This is accomplished by recirculating a portion of the fluid by means of a recirculating pump. For

Patented June 2, 1964 a more detailed description of such a recirculating system attention is directed to US. Patent No. 3,134,252 of June 2, 1964, in the name of Willburt W. Schroedter entitled Recirculation System for Steam Generator.

In vapor generators of this modified once-through flow type, it has been the practice of providing a recirculating pump system furnishing a positive head when operating within a predetermined range of the vapor load for the purpose of recirculating a portion of the fluid around a furnace section of the heating surfaces. Above such range, when recirculation is not required, the pump is .merely permitted to idle, with the pressure head of the pump system changing to a negtive value as the maximum operating capacity of the vapor generator is approached. For maintenance purposes provision of a reserve pump is desirable and also a bypass conduit bypassing both recirculating pumps, with the reserve pump conduit and the bypass conduit being arranged in parallel flow relation with the operating or main recirculating pump conduit. A positive or negative pump system as herein above referred to is defined as one having a higher or lower pressure head, respectively, at the common outlet point of the three parallel conduits, than at the common inlet point of the three parallel conduits. A stop and check valve is provided in the bypass line and in each pump line at the downstream side of the pumps. The pump stop valves are provided for isolating the pumps for maintenance purposes, and the bypass stop valve is provided for stopping flow through the bypass in case of failure of the check valve action. The check valves are so arranged that flow is only permitted in a downstream direction, i.e., in the direction from the suction side to the pressure side of the operating recirculating pump. When operating in the higher load range and above the load point at which recirculation ceases, and with a negative head on the main recirculatingpump system, the check valve in the bypass line as well as in the reserve pump line will be open and permit parallel flow throughout the thre econduits. Accordingly, flow is established simultaneously through the main recirculating pump, through the reserve recirculating pump and through the bypass line. It has been found that when operating under these conditions the roating speed of the spare recirculating pump which is usually of the submerged motor type is insuflicient to provide the water pressure necessary for suitable and adequate lubrication of the pump bearings by water over an extended period of time, since a minimum speed is required to properly lubricate a pump of this type. Therefore, when these pumps, which may be of the wet motor or the canned motor type, are subjected to the conditions of operation at higher loads without recirculation and with a negative head at the recirculating pump system, bearing difficulties will develop and the pump must be taken out of service for repair at uneconomically frequent intervals.

It has also been found when operating with the conventional recirculating pump system having a negative head characteristic at higher loads, that the check valves in the bypass line and in the reserve recirculating pump line are forced to remain open for the long periods during which the vapor generator normally operates in the higher nonrecirculating load range. Experience indicates that the possibility of a check valve sticking in the open position is greatly increased when such a valve remains open for any length of time. Operation of the unit with the check valves in the bypass line and spare pump line open is therefore highly undesirable.

The invention discloses an improved pump system and method for recirculating the working fluid, whereby the above disadvantages are considerably reduced or eliminated.

Referring now to FIG, 1, fuel and air, are introduced into a furnace by way of burners 12 for burning. The walls of furnace 10 are lined with heat-absorbing fluid-carrying tubes 14 arranged in parallel relation. The hot combustion gases are cooledby transfer of heat to these tubes for the generation of vapor. They are further cooled by passing in succession over high temperature superheater 16, low temperature superheater 18 and economizer 26. The combustion gases may in addition be cooled by transfer of heat to other heating surfaces such as a reheater and an air heater, not shown, before they are dispersed into the atmosphere by way of an insel 26, wherein the relatively cool fluid is mixed with the recirculated portion of the working fluid having a higher temperature.

After mixing in vessel 26 the fluid passes through one or more of three parallel conduits 28, 30, and 32 to a furnace wall inlet header 34, through furnace wall tubes 14 and into outlet header 36. Recirculating pumps 38 and 40 are provided in conduits 3i) and 32, respectively, with conduit 28 serving as a bypass for bypassing fluid around the recirculating pumps 38 and 40, one of which serves as the main recirculating pump and the other as a spare or reserve recirculating pump. There are also provided stop and check valves 42, 44, and 46 in bypass line 23 and recirculating pump lines 30 and 32, respectively. These valves permit flow in the direction indicated by the arrows but prevent flow in the opposite direction.

Having passed through the furnace wall tubes l -i and into outlet header 36 a portion of the Working fluid returns via recirculating duct 4-8 to mixing vessel 26' during operation of the vapor generator within a predetermined range of vapor load or vapor requirement. As earlier described the returned and heated fluid is mixed in the vessel 26 with the relatively cool fluid having passed through the economizer 20. As stated herein above, it is desirable during the above-mentioned predetermined lower operating range to recirculate a portion of the fluid around the furnace tubes 14 for the purpose of maintaining a,.

minimum permissible velocity in these tubes to prevent overheating. A check valve 49 is provided in the recirculating conduit 48 to prevent flow therethrough which would bypass heating tubes 14.

While a suitable amount of the working fluid is thus returned to the mixing vessel 26 the remaining portion thereof passes through conduit 50 to low temperature snperheaterls, through conduit 52 into high temperature superheater 16 and thereafter through conduit 54 to a point of use such as turbine 56.

In accordance with the invention the disadvantages of a conventional recirculating system as earlier described are reduced or eliminated by employing recirculating pumps 38 and 46 which are organized to maintain a positive head across the pump system at all loads, even when operating in the higher load range when recirculation of the fluid may not be required. In order to simplify description of the invention, it is assumed that mixing vessel 26 constitutes the common inlet point of the three parallel conduits 28, 39, and 32 of the pump system shown in FIGS.

- 1 and 4, and inlet header 34 the common outlet point of the three parallel conduits 28, 36, and 32.

if the generator load below which recirculation is desired and above which such recirculation ceases is assumed to be 85 percent of maximum load, the feed pump 22 will force the working fluid through the tubes of economizer 20 and deliver the fluid to the inlet of the recirculating pump 38 or 4th at a suitable pressure corresponding to the 85 percent of vapor requirement. Assuming that the operating pump is pump 40 and the idle pump is pump 38, then the positive head produced by the recirculating pump 40 is added to this pressurefor the purpose of forcing the fluid through the furnace wall tubes 14. A drop in pressure takes place while the fluid passes through these tubes. However, at the assumed percent load a pressure is retained at the outlet of the furnace walls such as at header 36 which is substantially of the same value as the pressure at the mixing vessel 26. Under these conditions recirculation ceases and fluid flow through conduit 48 stops.

When operating below theassumed 85 percent of load which load point is also called the recirculation run-out point, the pressure at the outlet of the furnace wall tubes is maintained in excess of the pressure at the upstream side of the recirculating pump 40 or at the mixing vessel 26. Consequently, recirculation is induced and is taking place. v 6

During operation above the assumed 85 percent of maximum load, the pressure at the furnace wall tubes outlet or at header 36 is lower than the pressure at the upstream side of the recirculating pump 40 or the mixing vessel 26. Accordingly, no recirculation is taking place.

In FIG. 2 a wet motortype recirculating pump is shown by way of example to illustrate one of the submerged motor type pumps. A pump of the canned motor type which is another submerged motor type pump may also be used in connection with the recirculating circuit shown in FIG. 1. Such a pump presents the same problems as far as water lubrication of the bearings is concerned.

In the pump 38, 40 illustrated in FIG. 2 the working fluid enters the impeller housing 60 by way of pump inlet 62 and is discharged through pump outlet 64. A water submerged motor 66 mounted on shaft 68 is rotatably supported in bearings 70, 72, and 74. A pump impeller 77 is provided at one end of shaft 68 and a coolant im- -88 serves to reduce the temperature of the lubricating water. A minimum rotational speed of the pump is required to provide suflicient flow of lubricating water at the necessary temperature and viscosity for the proper forming of a thin water film between the sliding surfaces of the bearings. If the pump is rotating at speeds below this minimum speed bearing difliculties develop which may lead to a shut-down of the vapor generator.

Attention is now called to FIG. 3, for an explanation of the conditions which give rise to these difiiculties when operating with a pump system having a negative head at vapor generator loads approaching maximum capacity and above the recirculation run-out point. The curves of FIG. 3 indicate flow resistance and pump system head when plotted. against fluid'fiow. Curve A shows the positive pressure portion of the pump system head characteristic of a recirculating pump system as heretofore practiced in the prior art. A positive head is maintained at the common outlet point 34 with respect to the common inlet point 26 (see FIG. 4) and throughout a major lower portion of the vapor requirement range as indicated by curve A above the zero pressure line P and ending at point 90, the point of intersection of curve A with the zero pressure line P. This point corresponds to a vapor load above the recirculation run-out load which latter is indicated by line 91. At loads above the point 90 load the pump system produces a negative head at the common outlet point or header 34 with respect to the common inlet point or vessel 26. This negative head portion of curve A marked A1 results from the fact that the Working fluid now flows through all three parallel lines or branches, the bypass line 28, the reserve pump line 30 and the main pump line 32. The flow quantity passing through bypass line 28 is indicated by curve A2, that passing through reserve pump line A3, and that passing through the main pump line only by curve A4, with curve A1 representing the sum total of all three flow quantities A2, A3, and A4.

The efiect of operating in this manner as heretofore practiced in the prior art is two-fold:

First, since the pressure above point 90 is less at the common outlet point 34 than at the common inlet point 26, see FIG. 4, fluid flow through lines 28 and 30 takes place with the check valves 42 and 44 (see FIG. 4) open during operation of the vapor generator in the higher load range above that indicated by point 90.

Second, with the working fluid passing through line 30, the reserve pump 38 is being driven by this flow and rotates at a speed which is considerably below that required for a satisfactory lubrication of the pump bearings 70, 72, and 74, see FIG. 2, as earlier described.

Since the vapor generator is operated most of the time at maximum load, the above three-way flow of the fluid through branches 28,. 30, and 32 will take place throughout long operating periods. Because of the likelihood of a check valve sticking in the open position, long-term operation with the valve open should be avoided whenever possible. Furthermore, operation of a submerged motor type pump when the pump merely floats on the line for an extended period of time should also be prevented, if at all possible, because of the damage that may be caused to the water lubricated bearings.

. To avoid the above disadvantages the invention provides a recirculating pump system which maintains a positive head throughout the entire operating load range.

This is indicated in FIG. 3 by curve B which intersects the zero pressure line P at a point 92 located Well above the maximum vapor requirement line 93. In this manner a positive pressure head is established at common outlet point or furnace inlet header 34 above that prevailing at common inlet point or mixing vessel 26, and fluid flow through the parallel branches 28 and 30 is prevented by the check valves 42 and 44 which are now closed.

An additional advantage is derived from the invention by the fact that when operating with a recirculating pump system having a positive pressure characteristic at higher loads and above the load when recirculation ceases, the recirculating pump 40 acts as a booster pump for the feedwater pump 22 at these higher loads. The flow losses which would be introduced with the recirculating pump idling on the line and actually producing a negative pressure head, are thereby completely eliminated.

This is conveniently illustrated in FIGS. 5 and 6 and in conjunction with FIG. 4 showing various elements of the vapor generating system in line with the corresponding points on the sets of curves shown in FIGS. 5 and 6.

The curves of FIG. 5 illustrate the pressure drops and rises experienced by the working fluid when passing through the system, and when employing a so-called negative head pump as heretofore practiced in the art. The curve of FIG. 6 shown similar pressure drops and rises of the Working fluid when operating with a so-called positive head pump as disclosed by the present invention.

In FIGS. 5 and 6 curves C and C1, respectively, shown in solidlines, illustrate, at the vapor generating load when recirculation ceases, the pressure drops and rises as the fluid passes from the feed pump 22 at the extreme left through the various pipe lines and heating surfaces and recirculating pump to the vapor turbine 56 shown at the extreme right, with the pressure at point 26a (mixing vessel 26) being equal to the pressure at point 36a (outlet header 36), and a rise in pressure due to the recirculating pump 40 as indicated by points 94 and 95.

Curves D and D1, shown in dot and dash lines, illustrate the pressure drops and rises as the fluid passes through the system at vapor load requirements below the circulation run-off point, and when recirculation is established due to the higher pressure maintained at point 36b (outlet header 36) than at point 26b (mixing vessel 26).

Curve E and E1 in FIGS. 5 and 6, respectively, shown in dash-dash lines, illustrate the pressure drops and rises .the pressure drop and rise conditions as they exist in an organization equipped with the improvided positive pressure pump system as disclosed by the invention, at rise occurs in the recirculating pump 40 from point 8 to point 99. Accordingly the recirculating pump 40 does not contribute to the pressure losses of the system at top loads, but instead acts as a booster pump for the feed pump 22 Within the higher loads at which the unit is operating most of the time.

From the aforesaid it can readily be seen that three distinct advantages are derived from the invention.

First, a positive pressure head continually maintained during operation in the recirculating pump system will keep the check valves in the bypass line and in the reserve recirculating pump line closed throughout the entire load range. Thus, these valves are likely to remain in a perfect operating condition when it becomes necessary to put the reserve pump into operation or to make use of the bypass line.

Second, employing a recirculating pump system having a positive head throughout the entire load range will prevent idling of the reserve pump at rotational speed below that required for proper maintenance of water flow through the Water lubricated bearings.

Third, the use of a recirculation pump system having a positive head throughout the operating range of the unit will permit this pump to act as a booster pump in cooperation with the feed pump at the higher and most frequent loads thereby reducing the power requirements of the feed ump. p The invention is applicable to various arrangements of recirculating circuits regardless of the location of the recirculating pump in the circuit, provided the recirculating pump is pumping both the through-flow as well as the recirculated-flow of the working fluid.

We claim:

1. In a modified once-through flow vapor generator operating at, below and above a predetermined load point, and having a furnace for producing hot combustion gases, the method of providing straight-through flow of the working fluid through a continuous path of said fluid including several heating sections, when operating the vapor generator above said predetermined load point, and in addition, providing recirculated fluid flow around a second heating section of said path when operating below said predetermined load point, said second heating section having a fluid fiow resistance directly varying with load, the steps comprising: forcing a stream of working fluid through a first section of said path in heat absorbing relation with said gases to establish a first fluid pressure at the outlet end of said first section; thereafter while operating within a lower load range and up to said predetermined load point forcing said heated fluid through a second section of said path in heat absorbing relation with said gases by establishing a second fluid pressure higher than said first fluid pressure at the inlet end of said second section and a third fluid pressure higher than said first fluid pressure at the outlet end of said second section, by virtue of the pressure difference established between said first and third pressures recirculating a portion of said fluid from the outlet end of said second section to the outlet end of said first section when operating within said lower load range and up to said predetermined load point; and within the operating load range above said predetermined load point maintaining said second fluid pressure at a value which is higher than said first fluid pressure While in conjunction with the fluid flow resistance of said second section maintaining said third fluid pressure at a value which is lower than said first fluid pressure. I

2. In a modified once-through flow vapor generator operating at, below and above a predetermined load point, and having means for producing hot gases, the method of providing straight-through flow of the Working fluid through a continuous path of said fluid including first and second heating sections when'operating the vapor generator above, at and below said predetermined load point, and in addition, providing recirculated fluid flow around a second heating section of said path when operating below said predetermined load point, said second heating section having a fluid flow resistance directly varying with load; the steps comprising: forcing a stream of working fluid through a first section of said path in heat absorbing relation with said gases to establish a first fluid pressure at the outlet end of said first section; thereafter forcing said heated fluid through a second section of said path in heat absorbing relation with said gases by establishing a' second fluid pressure higher than said first fluid pressure at the inlet end of said second section; while operating within a lower load range and up to said predetermined load point establishing a third fluid pressure higher than 'said first fluid pressure at the outlet end of said second section, and thereby by virtue of the pressure difference established between said first and third pressures recirculating a portion of said fluid from the outlet end of said second section to the outlet end of said first section when operating within said lower load range and up to said predetermined load point; and within the operating load range above said predetermined load point maintaining said second fluid pressure at a value which is higher than said first fluid pressure while in conjunction with the fluid flow resistance of said second section maintaining said third fluid pressure at a value which is lower than said first fluid pressure,

3. In a modified once-through flow vapor generator operating at, below and above a predetermined load point, and having means for producing hot gases, the method of providing straight-through flow of the working fluid through a continuous path of said fluid including first and second heating sections connected by at least two parallely arranged portions of said continuous path when operating the vapor generator above, at and below said predetermined load point, and in addition, providing recirculated fluid flow around a second heating section of said path when operating below said predetermined load point, said second heating section having a fluid flow resistance directly varying with load; the steps comprising: forcing a stream of working fluid through a first section of said path in heat absorbing relation with said gases to establish a first fluid pressure at the outlet end of said first section; thereafter forcing said fluid through said twoparallelly arranged portions of said path that connect the outlet of said first section with the inlet of said second section; thereafter forcing said heated fluid through a second section of said path in heat absorbing relation with said gases by establishing a second fluid pressure higher than said first fluid pressure at the inlet end of said second section; while operating within a lower load range and up to said predetermined load point establishing a third fluid pressure higher than said first fluid pressure at the outlet end of said second section, and thereby by virtue of the pressure dilference established between said first and third pressures recirculating a portion of said fluid from the outlet end of said second section to 'the outlet end of said first section when operating within at a value which is lower than said first fluid pressure.

References Cited in the file of this patent UNITED STATES PATENTS 1,707,920 Norton Apr. 2, 1929 FOREIGN PATENTS 509,746 Great Britain July 20, 1939 831,175 Great Britain Mar. 23, 1960 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 l35 249 June 2 1964 I Willburt wa-schroedter et a1.

It is hereby certified that. errereppeers in the above numbered pateht requiring correction andthat the said Letters Patent should read as eorrectedxbelow.

vColulmn 2,1[1ine 2.) for "3 l3lt 252 read 3 l35 252 column 5,, line 55, for "curve of FIG; 6 shown read curves of FIGO, o sjhqw column 6 line l3 for "improvided" read --'improved-.if+; "line 1 4 #IEOI "at" read a Signed and sealed this 17th day of November 1964,

ikttest:

ERNEsT w; s;wT=eER I s EDWARD J. BRENNER Attesting Officer I v Commissioner of Patents 

1. IN A MODIFIED ONCE-THROUGH FLOW VAPOR GENERATOR OPERATING AT, BELOW AND ABOVE A PREDETERMINED LOAD POINT, AND HAVING A FURNACE FOR PRODUCING HOT COMBUSTION GASES, THE METHOD OF PROVIDING STRAIGHT-THROUGH FLOW OF THE WORKING FLUID THROUGH A CONTINUOUS PATH OF SAID FLUID INCLUDING SEVERAL HEATING SECTIONS, WHEN OPERATING THE VAPOR GENERATOR ABOVE SAID PREDETERMINED LOAD POINT, AND IN ADDITION, PROVIDING RECIRCULATED FLUID FLOW AROUND A SECOND HEATING SECTION OF SAID PATH WHEN OPERATING BELOW SAID PREDETERMINED LOAD POINT, SAID SECOND HEATING SECTION HAVING A FLUID FLOW RESISTANCE DIRECTLY VARYING WITH LOAD, THE STEPS COMPRISING: FORCING A STREAM OF WORKING FLUID THROUGH A FIRST SECTION OF SAID PATH IN HEAT ABSORBING RELATION WITH SAID GASES TO ESTABLISH A FIRST FLUID PRESSURE AT THE OUTLET END OF SAID FIRST SECTION; THEREAFTER WHILE OPERATING WITHIN A LOWER LOAD RANGE AND UP TO SAID PREDETERMINED LOAD POINT FORCING SAID HEATED FLUID THROUGH A 